Technical Field
The invention relates to the removal of solvents from polymer formulations. More particularly, the invention relates to devolatilization systems that are used to finish formulations of certain polymers. Even more particularly, the invention is directed to an improved polymer solution concentrator machine and accompanying devolatilization process which enable concentration of polymer solutions to higher concentrations and viscosities than conventional equipment, overcome the viscosity limits and metering difficulties of the prior art, reduce the overall size of the devolatilization system when compared to the prior art, and reduce the energy, resources and cost required to finish polymer formulations.
Background Art
Reference herein shall be made to the terms polymer and polymer formulation with the understanding that such terms include polymers and polymer formulations as known to those skilled in the art, elastomers and elastomer formulations as known to those skilled in the art, and combinations thereof. In the formulation or production of certain polymers, the polymers synthesized are polymerized in a dilute solution of solvent, typically between 5 and 40 weight percent (wt %) polymer to solvent. After polymerization, in order for the polymer products to be prepared for further use, the solvents used during formulation must be substantially removed. Usually, a finished polymer with about 0.03 percent (%) solvent by weight or less is desired. The removal of solvent is often performed by a devolatilization process, which will be described below.
In the art, the devolatilization or removal of formulating solvents from polymer formulations is generally achieved through a two-part process. The first part or step of the process is an initial devolatilizing step. For certain materials, such as thermoplastic rubber (TPR), the polymer formulation is fed into a flash tank, in which solvent vapor separates from the polymer formulation based on the heat of the solution. For other materials, such as elastomer-based formulations, steam stripping is employed, in which large quantities of steam are employed to strip or drive off the solvent in a series of strippers or tanks, to create a water slurry. The second part or step is to isolate the polymer from the remaining solution or water slurry using an extrusion process.
When steam stripping is employed in the initial step of solvent removal, the polymer formulation is transferred from a reaction vessel by a series of pumps to a series of strippers or tanks. Steam is employed in the tanks to drive off the solvent, creating a water slurry. The water slurry is fed to an extruder to isolate the polymer from the remaining solution. The steam stripping process is the most widely used process in the prior art for isolating solution polymer from the solvent. It has the significant drawbacks of using very high energy consumption to generate the steam that is required for the process, and of generating significant undesirable emission levels. Because of these drawbacks, industry has long sought to remove the solvent without the use of steam. It has been predicted that energy savings in a range of about 60% are possible for a process that does not create a water slurry to finish the polymer.
When a flash tank is employed in the initial step of solvent removal, the process is often referred to in the art as a direct devolatilization process. In the direct devolatilization process, the polymer formulation is transferred from a reaction vessel by a series of pumps through a pre-heater element. The pre-heater elevates the formulation temperature, increasing the vapor pressure of the solvents in preparation for the initial removal of solvents. The formulation is then transferred into to the top of a flash tank, where it is gravity fed into the heat-controlled tank interior. The reduction in pressure from the super-heated solution to the lower flash tank pressure causes the solvent to vaporize and separate from the polymer solution, as is known in the art. The resulting vapor is vented out of the flash tank through a vapor takeoff located on the top of the tank. The remaining concentrated formulation collects at the bottom of the flash tank. More particularly, due to the flashing of solvent from the dilute formulation in the flash tank, the remaining formulation which collects at the bottom of the flash tank is more concentrated than when entering the tank.
The concentrated formulation that collects at the bottom of the flash tank is pumped by a gear pump to an extruder. The extruder then isolates the polymer from the solvent by evaporating the remaining solvent from the formulation as known in the art.
In conventional devolatilization processes that employ a flash tank, the initial step of solvent removal utilizing a flash tank is limited by the flash tank design. The polymer solution must flow by gravity to the discharge device at the bottom of the tank. However, concentrated polymer solutions have very high viscosity under the flash conditions desired, for example at 60-90 wt % polymer and temperatures between 60 and 120 degrees Celsius (° C.). Because the formulation flows by gravity within the flash tank, the concentration of the formulation typically must be controlled to a level which maintains a low enough viscosity of the formulation for it to properly flow down the inner wall of the vessel. If the viscosity of the formulation is too high, as is the case with an over-concentrated formulation, the polymer in the formulation can potentially adhere to the side of the vessel and turn dark in color or even cross-link in the vessel, resulting in an undesirable product.
Additionally, for gravity feeding, a level of polymer formulation must be maintained on top of the gear pump in order to provide the required net positive suction head pressure to keep the discharge device flooded. In the case of an over-concentrated formulation, this level must increase with the increasing viscosity of concentrated formulation to maintain proper function of the discharge device. As a result, the residence time of the polymer formulation in the tank must increase, which can create the potential for further over-concentrating of the formulation due to the conditions in which flashing is carried out within the tank. Furthermore, if the production line is stopped for any reason, over-concentration within the flash tank can potentially occur. Such over-concentration of the polymer formulation can result in the inability of the gear pump to function properly, which may require the over-concentrated formulation to be manually removed, resulting in lost time and lost product.
Because of the concentration limitations during flashing in the initial step of solvent removal using conventional methods, subsequent devolatilization of the polymer product by extrusion or direct evaporation is more problematic because the polymer concentration must be reduced to allow a favorable discharge performance from the flash tank. However the lower concentration causes difficulties in the extruder such as entrainment of polymer in the vapor stream of the vents, unstable conveying performance, high residual solvent levels in the product, and low output. Moreover, due to their large size, flash tanks take up a great deal of valuable space. The direct devolatilization process has not been widely adopted because of the lack of equipment to handle the solution from a low viscosity, high volume to a very high viscosity, low volume concentrated solution.
As a result, there is a need in the art for a polymer solution concentrator machine and accompanying devolatilization process which enable concentration of polymer solutions to higher concentrations and viscosities than conventional equipment, overcome the viscosity limits and metering difficulties of the prior art, reduce the overall size of the devolatilization system when compared to the prior art, and reduce the energy, resources and cost required to finish polymer formulations. The improved polymer solution concentrator machine and devolatilization process of the present invention satisfy these needs, as will be described in detail below.